The present invention relates to a process for producing diorganopolysiloxane gums of low and high molecular weight from cyclic siloxanes and more specifically the present invention relates to a process for producing high molecular weight diorganopolysiloxane gums from cyclic tetrasiloxanes, where at least one of the organo groups appended to the silicon atoms in the diorganopolysiloxane has three carbon atoms or more.
In the process for producing low and high molecular diorganopolysiloxane gums having a viscosity that varies from 1,000 to 200,000,000 centipoise viscosity at 25.degree.C, wherein the high molecular weight gums are the basic ingredient for producing heat vulcanizable silicone rubber compositions, it is often desirable to utilize cyclicsiloxanes. In such a process for producing diorganopolysiloxane gums and oils, the usual procedure is to take diorganochlorosilanes of high purity, hydrolyze such diorganodichlorosilanes in water at about room temperature, take the hydrolyzate and separate the water from it, and then add to the hydrolyzate a catalyst such as, potassium hydroxide or sodium hydroxide. The resulting mixture is heated at elevated temperatures of above 100.degree.C for a period of time of anywhere from 1 hour to 8 hours so as to distill overhead and obtain from the hydrolyzate a large proportion of cyclic trisiloxanes, cyclic tetrasiloxanes, cyclic pentasiloxanes and etc. In this cracking procedure of the hydrolyzate which results in the production of a large amount of cyclic siloxanes from the hydrolyzate, it is desirable to produce and separate from the cyclic siloxanes that are formed the cyclic tetrasiloxane. It has been found with methyl substituent groups that such tetrasiloxanes may then be taken in substantially pure form and equilibrated in the presence of a catalyst such as, potassium hydroxide. In such an equilibration mixture, there is also present chain-stoppers, that is, disiloxanes and/or low molecular weight diorganopolysiloxanes containing triorganosiloxy terminal groups, wherein the triorganosiloxy terminal groups function as the chain-stopper. Such tetrasiloxanes are equilibrated with such chain-stopper in the presence of small amounts of potassium hydroxide at elevated temperatures above 150.degree.C to result in equilibration mixture wherein about 85% of the cyclic tetrapolysiloxanes are converted into the desired low or high molecular weight diorganopolysiloxane gum or oil of anywhere from 1,000 to 200,000,000 centipoise viscosity at 25.degree.C.
At this equilibration point, as much of the tetrasiloxanes are being formed into the diorganopolysiloxane oil or gum as there is of the already formed diorganopolysiloxane oil or gum breaking down and reforming cyclic tetrasiloxanes. It has been found in this equilibration procedure that at most only 85% of the original cyclic tetrasiloxanes can be converted to the desired diorganopolysiloxane gum or oil, with the other 15% by weight of the cyclic tetrasiloxanes being present in equilibration with diorganopolysiloxane gum or oil.
At this point, the catalyst is neutralized and the volatiles are removed to result in the desired diorganopolysiloxane oil or gum. With this procedure it has been found it is possible to obtain a diorganopolysiloxane oil or gum of anywhere from 1,000 to 200,000,000 centipoise viscosity at 25.degree.C, but only where the organo substituent groups are methyl, vinyl or phenyl. When it has been attempted to form diorganopolysiloxane oils or gums where at least one of the organo groups appended to the silicon atom is an aliphatic radical or halogenated aliphatic radical of three carbon atoms or more, it was found that tetra cyclicsiloxanes and higher cyclic siloxanes such as, pentacyclic siloxanes would not work in such an equilibration procedure as that described above. It was found that with such cyclic tetrasiloxanes containing at least one organo substituent group on the silicon atom which was an aliphatic or haloaliphatic radical of three carbon atoms or more that at the equilibration point there would be a very low yield of diorganopolysiloxane oil or gum, that is, only 10 to 20% of the cyclic tetrasiloxane or higher cyclic siloxane would be converted to the diorganopolysiloxane gum or oil, versus the 85% by weight which is experienced with octamethyltetrasiloxanes. Thus, it is necessary to find a procedure wherein low molecular weight or high molecular weight diorganopolysiloxane gums or oils could be formed where one of the organo groups appended to the silicon atoms was an aliphatic or haloaliphatic radical of three carbon atoms or more and wherein such diorganopolysiloxane gums or oils can be formed in high yield from cyclic tetrasiloxanes.
In this respect, note the disclosure of Pierce et al, U.S. Pat. No. 2,979,519, Column 2, beginning with line 6, where it is stated that it has been found that commercially successful rubbers, that is, heat vulcanizable silicone rubbers, cannot be prepared by known methods from the crude hydrolysis product of chlorosilanes of the formula, ##EQU2## or from the cyclic siloxanes of the formula, ##EQU3## where x is 4 or more. It should also be noted that this statement is supported by affidavit evidence in the file history of this patent. It is stated in the file history of that patent that diorganopolysiloxane gums of high molecular weight cannot be formed from cyclic tetrasiloxanes where one of the substituent groups in the cyclic polysiloxane contains 3 carbon atoms or more and specifically contains a CH.sub.2 CH.sub.2 R group, where R is a perfluoroalkyl radical.
It was the contention of Pierce et al in U.S. Pat. No. 2,979,519 and its file history, that high molecular weight diorganopolysiloxane gums that allegedly would not be formed from such tetrasiloxanes could be formed from cyclic trisiloxanes.
With respect to the formation of low molecular weight polysiloxane oils having a low viscosity and having the R, R' substituent groups, it has been found that such oils could be formed from cyclic tetrasiloxanes only in low yield such as 10% to 15%.
In addition, the reaction equilibration mixture was such that most of it was still composed of cyclics rather than low viscosity desired polymers when the equilibration reaction was terminated.
It should also be understood that the process of Pierce et al in U.S. Pat. No. 2,979,519 could not be used to form such low molecular weight polysiloxanes from cyclic trisiloxanes since such cyclic trisiloxanes immediately react to form high molecular weight polymers so that even with a large amount of chain-stoppers in the reaction mixture, the reaction cannot be controlled to form low molecular weight polymers.
Accordingly, it is highly desirable to develop a process for the production of low molecular weight polysiloxanes of a viscosity varying from 1,000 to 200,000 at 25.degree.C, which polysiloxane is obtained in high yield as 70% or more by the equilibration of cyclic tetrasiloxanes of Formula (1).
One difficulty with the Pierce et al procedure in the use of the cyclic trisiloxanes which is set forth in the foregoing patent is that such cyclic trisiloxanes are formed in low yield during the initial cracking of the hydrolyzate with potassium hydroxide or sodium hydroxide. Accordingly, the amount of cyclic trisiloxane that is formed during the cracking procedure is not as high as the tetra cyclicsiloxane. Accordingly, various procedures are utilized to maximize the yield of cyclic trisiloxanes from the cracking process so as to make the use of such cyclic trisiloxanes to form high molecular weight polysiloxanes as economic as possible. Nevertheless, in spite of such procedures, the process for forming polymers from cyclic trisiloxanes is still more expensive than from the use of cyclic tetrasiloxanes.
It has now been unexpectedly found that at certain low temperature ranges which were not envisioned previously and in the presence of certain select catalysts, cyclic tetrasiloxanes and mixtures of such cyclic tetrasiloxanes can be equilibrated at relatively high yield and where one of the organo substituent groups appended to the silicon atom is an aliphatic or haloaliphatic radical of 3 carbon atoms or more such as, the --CH.sub.2 CH.sub.2 R radical, where R is a perfluoroalkyl radical. Such cyclic tetrasiloxanes can be equilibrated at relatively high yields to produce low molecular weight oils or high molecular weight diorganopolysiloxane gums suitable for forming heat vulcanizable silicone rubber compositions.
It is, thus, one object of the present invention to provide a process for producing low molecular weight oils or high molecular weight diorganopolysiloxane gums wherein one of the organo groups appended to the silicon atom is an aliphatic or haloaliphatic radical of three carbon atoms or more from cyclic tetrasiloxanes and mixtures of cyclic tetrasiloxanes.
It is an additional object of the present invention to provide for a process for producing low molecular weight oils or high molecular weight diorganopolysiloxane gums in high yield wherein one of the organo groups appended to the silicon atom is an aliphatic or haloaliphatic radical of at least three carbon atoms or more.
It is still another object of the present invention to provide for a process for producing low molecular weight oils or high molecular weight diorganopolysiloxane gums having a viscosity from 1,000 to 200,000,000 centipoise at 25.degree.C, where at least one of the organo groups appended to the silicon atoms is an aliphatic or haloaliphatic radical of 3 carbon atoms or more by equilibrating cyclic tetrasiloxanes and mixtures of cyclic tetrasiloxanes at low temperatures in the presence of certain select catalysts.
It is yet an additional object of the present invention to provide a process for forming diorganopolysiloxane oils or gums having a viscosity from 1,000 to 200,000,000 centipoise at 25.degree.C, where at least one of the organo groups appended to the silicon atoms is a --CH.sub.2 CH.sub.2 R.sup.5 substituent group, where R.sup.5 is a perfluoroalkyl radical, by reacting cyclic tetrasiloxanes.